The present invention relates to a method and apparatus for maintaining a constant magnetic field environment for a device the performance of which is adversely affected by time varying and static extraneous magnetic fields, and more particularly for reducing interference with the magnetic field of a magnetic resonance imaging (MRI) system.
In a typical MRI system, a magnetic structure is provided for developing a strong magnetic field within a gap within which a patient or other body is placed for examination. Typically, the magnetic field strength in the patient receiving gap is greater than 300 gauss. In addition to a strong magnetic field, medical magnetic resonance studies require a magnetic field stability of the order of a few parts per million.
Often it is necessary to locate an MRI system in areas that have changing environmental magnetic fields. For instance, the system may be located near a conductor carrying large currents which conductor is surrounded by a strong magnetic field. This field impacts upon the field produced by the magnetic structure of the MRI machine. The third rail or overhead wire of an electrical railway is an example. If the current through the conductor remains constant, the MRI machine can be calibrated to allow for the field produced by the conductor. However, if the current through the conductor varies for any reason, the changing magnetic field must be compensated for in order to maintain a desired level of MRI machine performance. A previous method for maintaining a constant value of the environmental magnetic field involves placing a measuring device such as a gauss meter in the near environment of the magnetic structure although out of the path of any leakage flux. The signal produced by the gauss meter is used to drive a large two-coil assembly where the MRI device magnetic structure is located between the two coils. The current in the two coils is varied until no variation of the magnetic field can be detected in the MRI magnet gap. Unfortunately, this method has many limitations. First, the gauss meter probe cannot be placed in the same space as the MRI magnet because the fringe field of the MRI magnet would saturate the probe. Second, the gauss meter provides essentially an open loop control system which requires constant re-calibration to maintain a constant field in the MRI magnet gap. Finally, the sensitivity of the gauss meter is not adequate for the resolution desired.